Pulp

In Pulp 2, users can get a list of installed plugins at the `/pulp/api/v2/plugins/types/` endpoint.
http://docs.pulpproject.org/dev-guide/integration/rest-api/server_plugins.html

For Pulp 3, it probably makes more sense to add this to the `v3/status/` endpoint, possibly only in "verbose" mode.

Currently, this feature is listed as 3.1+, but I think this feature would be very helpful as a sanity check for plugin writers to test the discovery of a bootstrapped plugin (ideally, freshly generated by our plugin template).
https://pulp.plan.io/projects/pulp/wiki/31+_Ideas_(post_MVP)#Status-API

Another reason to do this soon is to write reusable code that would be helpful in filling out the REST API with views for master ViewSets. For example, there is no endpoint at `v3/repositories/myrepo/importers/`, but it would be nice to see what importer types are available. I'm marking #2948 as related for this reason.

One idea for how an implantation might work is to keep a list of plugin_configs in our settings.py, similar to the INSTALLED_APPS list: https://github.com/pulp/pulp/blob/3.0-dev/platform/pulpcore/app/settings.py#L63.